In old conventional Prior Art used cooking of chemical cellulose pulp with continuous digesters it was common to use a pre-treatment arrangement with a chip bin, steaming vessel and an impregnating chip chute, before the cooking process is established in the digester. Steaming has been carried out in one or several steps in the chip bin, prior to the subsequent formation of a slurry of the chips in an impregnation fluid or a transport fluid. The steaming has been considered to be absolutely necessary in order to be certain of expelling the air and water that is bound in the chips, such that the impregnation fluid can fully penetrate the chips, and such that air is not drawn into the system.
Attempts have been made to integrate the chip bin with the impregnation vessel in order to obtain in this manner a simpler system.
Already in U.S. Pat. No. 2,803,540, a system was revealed in which the chips from a chip bin were fed to a vessel in which a combined steaming and impregnation was achieved. In this vessel, the chips were steamed at the upper part of the vessel and impregnation fluid at the same temperature was added at various levels in the vessel using distributing annular manifolds/headers located outside of the vessel wall, and having nozzles penetrating the wall. These principles were also applied in a process known as “Mumin cooking”, which is described in“Continuous Pulping Processes”, Technical Association of the Pulp and Paper Industry, 1970, Sven Rydholm, page 144. In this process, unsteamed chips were passed to a combined impregnation vessel, where steaming was obtained in the upper part, and to which impregnation fluid was added at a point in the upper part of the vessel during forced circulation. The impregnation fluid was in this case carried exclusively in the same direction of flow as the chips.
A similar system with a low pressure first common steaming and impregnation vessel is shown in U.S. Pat. No. 3,532,594, which also was put into operation for example at the Skoghall mill in Sweden. Here was heated impregnation liquid added via a central pipe to the chip volume, but also showing an additional central pipe for steam supply. This system was later abandoned due to various reasons such as run ability problems, capacity problems in subsequent feeding system and far too high reject and shive content in the blown pulp.
A system is shown in U.S. Pat. No. 5,635,025 in which the chips are fed without prior steaming to a vessel in the form of a combined chip bin, impregnation vessel and chip chute. Steaming of the chips takes place here, the chips lying above the fluid level, and a simple addition of impregnation fluid takes place trough the vessel wall below the liquid level.
A further such system is revealed in U.S. Pat. No. 6,280,567, in which the chips are fed without prior steaming to an atmospheric impregnation vessel in which the chips are heated by the addition of warm black liquor that maintains a temperature around 130-140 C. The added impregnation liquid is added via nozzles in a manner similar to what is shown in U.S. Pat. No. 2,803,540, i.e. using supply nozzles penetrating the wall of the vessel.
An alternative system is revealed by SE 523.850 in which pressurized black liquor is added to the upper part of the steaming vessel, whereby the black liquor after being subjected to a pressure reduction releases steam for the steaming process. The addition of the hot black liquor is made by a horizontal supply pipe penetrating the chip bed, and having a multitude of holes over the length of the pipe. Even if the distribution of hot black liquor is made over a larger (but not over the complete cross-sectional) area this solution is not advisable since the horizontal pipe may hinder the chip plug movement. The prior art has mostly used either a central pipe or annular distribution nozzles for the added impregnation liquid to the combined steaming and impregnation vessel. During the last few years the design capacity of new digester systems has been significantly increased, from typically 500-2000 ADt/24 h to production rates over 5000 ADt/24 h. As the design production rate increases the combined chip steaming and impregnation vessel diameter increases as well. The fact that the diameter of the vessel increases and in combination with using prior art technology, the added treatment liquor may not be evenly distributed over the entire cross-sectional area of the vessel resulting in a chemical concentration gradient.
SE 523.850 discloses another solution to improve the distribution but it will then instead introduce a great risk for chip plug blockage.
Another disadvantage with a single central pipe for adding the treatment chemicals in larger treatment vessels is that the flow of chemicals is high and would be disruptive for a uniform plug flow movement in the lower part of the vessel.
Other disadvantages with only one central pipe for adding treatment chemicals are
(1) the diameter of the central pipe becomes big in systems with high design capacity, which means that a hole with same diameter as the pipe is formed below the end of the central pipe which may continue all the way down to the bottom of the vessel, which in turn give rise to (chip and) liquor channeling,(2) that a huge quantity of the generated flash steam is introduced at one point, which could cause steam channeling around the central pipe due to the high velocity of the steam going counter-current the chip flow, which could result in steam blow-through of the chip pile.
A disadvantage with a distribution manifold outside of the vessel and supply nozzles penetrating the wall of the vessel for adding the treatment chemicals is, except additional requirement of control valves/instrumentation and pipes, thus increased cost, that it is difficult to completely distribute the chemicals from the shell side to the center of the vessel, thus there is an apparent risk for liquor channeling along the shell of the vessel. All these disadvantages may cause an uneven treatment of the chips, such that quite different pulp quality is produced from those plug flows being closest to wall or central pipe.
The technique with a common treatment vessel for steaming and impregnation at substantially atmospheric conditions is marketed by Metso Paper under the name of IMPBIN™. Several improvements of the concept have been patented in;                SE 518.738 (=U.S. Pat. No. 7,381,302), with impregnation liquids added at successively increasing temperature at positions in IMPBIN with higher static head;        SE 528.448 (=EP1818445), with liquor circulations of IMPBIN separated from those in digester;        SE 530.725 (=EP2065513), with cooling showers in top of IMPBIN for knocking down blow trough of malodorous gases.        
The above mentioned disadvantages with prior art addition of treatment liquors are made more obvious as sizes of these steaming and impregnation vessels of the IMPBIN type becomes bigger. The diameter of a typical cylindrical IMPBIN vessel for a digester system with capacity over 5000 ADT per day is well over 9 meter.
The principle object of the invention is to obtain an improved arrangement for the addition of treatment liquors to chips during the manufacture of chemical pulp in a continuous process using a down flow vessel where chips are descending down the vessel in a plug flow, which arrangement does not demonstrate the disadvantages that are associated with other known solutions as described above. This principal objective becomes more important in high capacity processes, with capacities of producing well over 4000 and as much as 6000 ADT/pulp per day, and where treatment vessels becomes huge and having diameters well over 9 meter.
A specific objective is to enable equal treatment of the wood material in the entire volume of the treatment vessel, minimizing variations in kappa number of the produced pulp and decreasing the amount of rejects (uncooked wood material) from the process. By an even initial distribution of alkaline black liquor, thus ensuring that the alkali concentration over the vessel's entire cross-section and volume will be sufficient, could also the chip volumes being exposed to extensive acidic pre hydrolysis be reduced, since acidic pre hydrolysis is negative for pulp strength properties and yield.
Another specific objective is to minimize channeling effects in the chip or wood material plug flow, which formations of channels or voids is almost impossible to avoid by using singular central pipes with large diameters according to conventional prior art. If such channels are formed inside of the treatment vessel, then most or a large part of the added treatment liquor may be by-passed the bulk flow of wood material intended to be treated in the treatment vessel.
Yet another specific objective is to decrease the steam velocity of the steam being flashed out from the hot treatment liquor, which per se reduces the risk for channeling and leakage of steam along the exterior surface of the supply pipes. By reducing the steam velocity will also the risk for blow-trough, i.e. steam being pushed up and through the entire chip volume, be reduced.
The invention can advantageously be used when cooking hard wood and softwood wood chips, bagasse and other annual plants.
The characteristics of the invention are defined by the independent claims, and optional embodiments are defined in dependent claims in order of dependency of preceding claims. The invention is also disclosed in a preferred embodiment, but any specific feature of this embodiment could as such be included in the invention optionally, if not specifically defined as a necessary feature for the argued effect.